Titanium or titanium alloy plates (hereinafter also representatively referred to as “titanium plate(s)”) have excellent corrosion resistance and satisfactory specific strength (specific intensity) and have been recently used as materials for exchangers and chemical processing plants. In particular, titanium plates have been widely used for heat exchangers using seawater, because they are free from corrosion by the action of seawater.
Plate-type heat exchangers are one of major applications of titanium plates. The titanium plates adopted to these applications desirably have such satisfactory press formability as to be formed into complicated shapes, for higher efficiency of heat transfer (heat-transfer efficiency). In addition, these titanium plates should have such high strengths as to allow the heat exchangers to be operated under higher operation pressure. However, strength and press formability are opposing properties, and no titanium plate satisfying the two properties has been obtained yet.
To improve press formability in metallic plates such as steel sheets, techniques are employed for improving the property typically by alloy design and structure control for optimizing, for example, the aggregate structure and grain size. In addition to these techniques, techniques for applying a lubrication film to the surface of a steel sheet are known, as disclosed typically in PTL 1 and PTL 2. The press formability is improved according to these techniques by forming the lubrication film on the surface of the steel sheet and thereby allowing the steel sheet to deform and to fit a die.
The respective techniques also indicate the application of the formation of a lubrication film to a titanium plate as the metallic plate. Independently, PTL 3 and PTL 4, for example, disclose that when a lubrication film is applied to a steel sheet and the original steel sheet is controlled to have a r value and an elongation at specific levels or higher, the lubrication film may exhibit effects. PTL 3 and 4 mention that the formability is generally improved with an increasing elongation and an increasing r value, and describe that a steel sheet with better formability can exhibit further better formability by applying a lubrication film to the steel sheet. However, the present inventors investigated on the influence of a lubrication film on press formability of a titanium plate and found that satisfactory formability is not always obtained by forming a lubrication film on the surface of a titanium thin plate which merely has a high elongation and a high r value and shows good formability.
Specifically, the titanium plate has a crystal structure of close-packed hexagonal lattice (hcp) and is known to have larger anisotropic aspect in properties thereof than that of steel sheets and other metallic plates. Titanium plates manufactured by rolling a material in one direction show properties which significantly differ between the rolling direction (hereinafter also referred to as “L direction”) and a direction perpendicular to the rolling direction (hereinafter also referred to as “T direction”). There are specific characteristics seen only in the titanium plates. Typically, the titanium plates have a yield strength (YS) in the L direction lower than that in the T direction by approximately 20% or more and have an elongation in the L direction higher than that in the T direction by approximately 40% or more. Probably owing to differences in characteristics between the titanium plates and the steel sheets, the techniques, which are believed to be effective for steel sheets, do not effectively exhibit their effects when merely applied to the titanium plates without modification.    PTL 1: Japanese Patent No. 3056446    PTL 2: Japanese Unexamined Patent Application Publication No. 2004-232085    PTL 3: Japanese Unexamined Patent Application Publication No. 2003-65564    PTL 4: Japanese Patent No. 3639060